<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        /** .parent {
            width: 400px; 
            padding: 30px; 
            border: 5px solid #000000; 
            box-sizing: border-box;
        }

        .child {
            width: 100%;
            padding: 20px;
            border: 2px solid red; 
            box-sizing: border-box;
        } 
        */
        .container {
  overflow: hidden; /* 清除浮动 */
}
.center, .left, .right {
  float: left;
  height: 300px;
}
.center {
  width: 100%; /* 占满父容器 */
  background: #fff;
}
.center-inner {
  margin-left: 200px; /* 给左栏留位置 */
  margin-right: 150px; /* 给右栏留位置 */
  height: 100%; /* 和外壳同高 */
}
.left {
  width: 200px;
  background: #f0f0f0;
  margin-left: -100%; /* 拽到左边 */
}
.right {
  width: 150px;
  background: #f0f0f0;
  margin-left: -150px; /* 拽到右边 */
}
    </style>
</head>
<body>
    <!-- <div class="parent">
        <div class="child"></div>
    </div> -->

    <div class="container">
  <div class="center"> <!-- 中间栏外壳 -->
    <div class="center-inner">中间栏内容（自适应）</div> <!-- 真实内容容器 -->
  </div>
  <div class="left">左栏（固定200px）</div>
  <div class="right">右栏（固定150px）</div>
</div>
</body>
</html>